Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A palladium-containing electroplating solution and method for providing a
palladium or palladium alloy membrane on a porous metal support are
provided. The subject invention uses electroplating to manufacture a
palladium or palladium alloy membrane on a porous metal with a decreased
preparation time and simplified preparation procedure. Moreover, the
palladium or palladium alloy membrane prepared by the subject invention
exhibits excellent compactness and good resistance to the hydrogen
embrittlement, as well as a high applicability.

Claims:

1. A palladium-containing electroplating solution, comprising:about 2 g/L
to about 200 g/L of palladium in palladium sulfate;about 10 g/L to about
200 g/L of a reactive conductive salt;about 10 g/L to about 150 g/L of a
complexing agent; andenough buffering agent to give the electroplating
solution a pH of about 9 to about 12.

2. The electroplating solution of claim 1, comprising about 5 g/L to about
50 g/L of palladium in palladium sulfate, about 70 g/L to about 150 g/L
of the reactive conductive salt, about 30 g/L to about 70 g/L of the
complexing agent, and enough buffering agent to give the electroplating
solution a pH of about 10 to about 11.

6. The electroplating solution of claim 1, wherein the complexing agent is
selected from a group consisting of potassium nitrate, ammonium citrate,
EDTA-Na2, EDTA-Na4, and combinations thereof.

7. The electroplating solution of claim 1, wherein the buffering agent is
a hydroxide.

8. The electroplating solution of claim 1, wherein the buffering agent is
selected from a group consisting of sodium hydroxide, potassium
hydroxide, ammonium hydroxide, and combinations thereof.

9. The electroplating solution of claim 1, further comprising sulfuric
acid in an amount sufficient for rendering the concentration of
SO.sub.4.sup.2- in the electroplating solution to be about 0.2 mole to
about 4 moles per liter.

10. The electroplating solution of claim 1, further comprising a second
metal salt other than palladium sulfate and selected from a group
consisting of: a copper salt, a silver salt, a gold salt, a nickel salt,
a platinum salt, an indium salt, and combinations thereof.

11. The electroplating solution of claim 10, wherein the second metal salt
is a copper salt selected from a group consisting of copper sulfate,
copper chloride, and combinations thereof and in an amount sufficient for
rendering the electroplating solution to contain about 0.2 g to about 100
g of copper per liter.

12. A method for providing a palladium or palladium alloy membrane on a
porous metal support, comprising:providing a porous metal support;
andelectroplating a palladium or palladium alloy membrane onto the metal
support with a palladium-containing electroplating solution, wherein said
palladium-containing electroplating solution comprises:about 2 g/L to
about 200 g/L of palladium in a palladium salt;about 10 g/L to about 200
g/L of a reactive conductive salt;about 10 g/L to about 150 g/L of a
complexing agent; andenough buffering agent to give the electroplating
solution a pH of about 9 to about 12.

13. The method of claim 12, wherein the porous metal support is composed
of stainless steel.

14. The method of claim 12, wherein the electroplating step is carried out
at an electroplating bath temperature ranging from about 40.degree. C. to
about 90.degree. C. under a current density ranging from about 0.01
A/dm2 to about 1.5 A/dm.sup.2.

15. The method of claim 12, wherein the metal support is rotated during
the electroplating step at a speed of not higher than about 1000 rpm.

16. The method of claim 15, wherein the metal support is rotated at a rate
ranging from about 100 rpm to about 500 rpm.

18. The method of claim 12, further comprising coating a medium layer on
the metal support prior to the step of electroplating the palladium or
palladium alloy membrane.

19. The method of claim 18, wherein the medium layer is coated onto the
metal support by an electroplating method and is composed of a material
selected from a group consisting of nickel, copper, silver, gold,
platinum, and combinations thereof.

20. The method of claim 12, wherein the electroplating step is a two-stage
electroplating step, one stage of which uses palladium sulfate as the
palladium salt and the other uses palladium chloride as the palladium
salt.

21. A composite with a palladium or palladium alloy membrane, comprising:a
porous metal substrate;a medium layer coated on a surface of the
substrate; anda palladium or palladium alloy membrane, coated on the
medium layer,wherein the palladium or palladium alloy membrane is
substantially free from exfoliation under a condition that the pressure
at the substrate side of the composite is up to about 3 absolute
atmospheres higher than the pressure at its palladium or palladium alloy
membrane side.

22. The composite of claim 21, wherein the porous metal substrate is
composed of stainless steel.

23. The composite of claim 21, wherein the medium layer is composed of a
material selected from a group consisting of nickel, copper, silver,
gold, platinum, and combinations thereof.

24. The composite of claim 21, wherein the palladium or palladium alloy
membrane is substantially free from exfoliation under a condition that
the pressure at the substrate side of the composite is up to about 5
absolute atmospheres higher than the pressure at its palladium or
palladium alloy membrane side.

25. The composite of claim 21, wherein the palladium or palladium alloy
membrane is substantially free from exfoliation under a condition that
the pressure at the substrate side of the composite is up to about 10
absolute atmospheres higher than the pressure at its palladium or
palladium alloy membrane side.

[0004]The subject invention relates to a palladium-containing
electroplating solution and a method for preparing a palladium or
palladium alloy membrane on a porous metal support by electroplating. The
method produces a palladium or palladium alloy membrane that is strongly
adhered to the porous metal support, thereby, providing a palladium
membrane tube fitting useful for the catalytic reactor during hydrogen
purification or synthesis.

[0005]2. Descriptions of the Related Art

[0006]A palladium or palladium alloy membrane can be prepared using the
electroless plating method, the vacuum sputtering method, or the
cold-rolled method. For plating a palladium or palladium alloy membrane
on a porous metal support, the electroless plating method is
conventionally used, such as that disclosed in Taiwan Patent Publication
No. 1232888 and U.S. Pat. No. 6,152,987. However, with the electroless
plating method, the adhesion to the membrane is dependent on the physical
adsorption of the chemically reduced metal particles on the substrate. As
a result, the temperature variation can exfoliate the palladium or
palladium alloy membrane from the porous metal support. Moreover, in
preparing a palladium membrane, the electroless plating method requires
multiple depositions (more than 6 to 7 times) to obtain a palladium
membrane with the desired thickness. Furthermore, the resulting membrane
is subjected to an annealing treatment for homogenization to complete the
preparation process. In addition, it is difficult to control the reducing
rates of the different cations (e.g., Pd ion, Cu ion, and Ag ion) and the
depositing rates. Therefore, multiple steps are required for reducing a
single ion and depositing the individual metal layers, and then, an
annealing step is carried out at a high temperature for a long time to
obtain an alloyed metal layer comprising two or more metals. In other
words, the electroless plating method is slow and results in poor
adhesion.

[0007]As mentioned above, the palladium membrane can be electroless plated
on a porous metal support. The palladium or palladium alloy membrane has
also been electroless plated on a porous ceramic support as disclosed in
Japan Laid-Open Patent Application No. 2002-119834 and No. 2002-153740.
Because the porous ceramic or glass support has a compact surface with
nano-sized pores (10-200 nm), the palladium or palladium alloy membrane
can easily block the pores, and thus, create a better plated membrane.
However, the porous support materials with nano-sized pores are expensive
and their manufacturing costs keep this product noncompetitive in the
market.

[0008]In the vacuum sputtering method for preparing a palladium or
palladium alloy membrane, the expensive vacuum apparatus and sputtering
targets involved therein also drive the manufacturing costs high and are
thus, undesirable in the market.

[0009]As for the use of the cold-rolled method for preparing a palladium
or palladium alloy membrane, the resulting membrane needs to adhere onto
the porous support in a specific way. Therefore, the procedures are
complicated, and the membrane suffers from poor adhesion and a low
manufacturing yield. Such a method is also unattractive.

[0010]The technology known at present for electroplating a palladium or
palladium alloy membrane is primarily applied to common supports with a
smooth surface mainly for the purpose of processing or decoration. For
example, the technology of electroplating a palladium or palladium alloy
membrane on a smooth surface is typically applied to ornaments such as
jewelry to prevent the decoloration due to the oxidation on their
surfaces or electronic components to improve the weldability. This
technology decreases the contact resistance, and enhances the
anti-oxidation properties. The resulting membrane has a thickness ranging
from about 0.3 mm to about 2 mm, such as that disclosed in U.S. Pat. No.
4,486,274. However, when the formulation of the palladium salt
electroplating solution employed in such known technology is applied to
the electroplating of porous metal supports, it is impossible to obtain a
compact palladium or palladium alloy membrane free of defects. In fact,
the resulting plated membrane has some pinholes thereon, rendering it
unsuitable for purifying elements used for supplying H2 with high
purity.

[0011]As a result, there is a common desire in the industry to provide a
method for preparing a palladium or palladium alloy membrane, with the
desired compactness and H2 permeability, on a porous metal support
in a more simple, timesaving and economical way.

SUMMARY OF THE INVENTION

[0012]One objective of the subject invention is to provide a
palladium-containing electroplating solution, which comprises palladium
sulfate, a reactive conductive salt, a complexing agent, and a buffering
agent.

[0013]Another objective of the subject invention is to provide a method
for providing a palladium or palladium alloy membrane on a porous metal
support, which comprises providing a porous metal support; and
electroplating a palladium or palladium alloy membrane onto the porous
metal support with a palladium-containing electroplating solution. Said
palladium-containing electroplating solution comprises a palladium salt,
a reactive conductive salt, a complexing agent, and a buffering agent.

[0014]Yet a further objective of the subject invention is to provide a
composite with a palladium or palladium alloy membrane, which comprises a
porous metal substrate; a medium layer coated on a surface of the
substrate; and a palladium or palladium alloy membrane coated on the
medium layer. The palladium or palladium alloy membrane is substantially
free from exfoliation under the condition that the pressure at the
substrate side of the composite is up to about 3 absolute atmospheres
higher than the pressure at its palladium or palladium alloy membrane
side.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 shows a flow chart of a method for electroplating a palladium
membrane onto a porous metal support in accordance with the subject
invention;

[0017]FIG. 3A shows an electron microscopic photograph of an electroplated
palladium alloy membrane in accordance with the subject invention;

[0018]FIG. 3B shows the composition analysis result of an electroplated
palladium alloy membrane in accordance with the subject invention;

[0019]FIG. 4 shows an electron microscopic photograph of a palladium
membrane electroplated using a two-stage electroplating treatment in
accordance with the subject invention; and

[0020]FIG. 5 is a schematic view of a palladium membrane shell and tube
reactor;

[0021]FIG. 6 is a photograph showing the result of a hydrogen
embrittlement test on a conventional electroless plated palladium
membrane;

[0022]FIG. 7 is a photograph showing the result of a hydrogen
embrittlement test on an electroplated palladium membrane of the subject
invention; and

[0023]FIG. 8 is a schematic view of a reactor utilizing the palladium
membrane of the subject invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024]The subject invention provides a palladium-containing electroplating
solution, which comprises palladium sulfate, a reactive conductive salt,
a complexing agent, and a buffering agent. In the electroplating
solution, there is about 2 g/L to about 200 g/L of palladium in the
palladium sulfate and preferably about 5 g/L to about 50 g/L. There is
also about 10 g/L to about 200 g/L of the reactive conductive salt and
preferably, about 70 g/L to about 150 g/L. There is about 10 g/L to about
150 g/L of the complexing agent and preferably, about 30 to about 70 g/L.
There is enough buffering agent to render the electroplating solution to
have a pH of about 9 to about 12, preferably about 10 to about 11.

[0025]In the palladium-containing electroplating solution of the subject
invention, the reactive conductive salt can provide conductive ions to
enhance the conductivity of the electroplating solution, so as to improve
the deposition efficiency and the quality of the palladium or palladium
alloy membrane. The reactive conductive salts suitable for the subject
invention comprise SO42- ion-providing compounds, and can also
be selected from a group consisting of salts of Group IA metals, ammonium
salts, and combinations thereof. When the SO42- ion-providing
compound is used as the reactive conductive salt, it can not only enhance
the conductivity of the electroplating bath, but also facilitate the
dissolution of palladium sulfate with a low solubility. Then, the
palladium concentration in the electroplating bath is increased to
further enhance the conductivity. For example (but not limited thereto),
the reactive conductive salt used in the subject invention can be
selected from a group consisting of sodium chloride, potassium chloride,
sodium sulfate, ammonium sulfate, ammonium chloride, sodium thiosulfate,
ammonium thiosulfate, ammonium citrate, and combinations thereof. The
preferred reactive conductive salt is ammonium sulfate.

[0027]The buffering agent in the palladium-containing electroplating
solution of the subject invention serves to decrease the deposition rate
of palladium. More specifically, as a noble metal, palladium has a
standard reduction potential of up to 0.997 V (i.e., the reducing
reaction occurs very quickly). Therefore, to control the entire
electroplating process, a buffering agent is normally added to the
electroplating solution to slow down the reducing reaction of the
palladium metal, so that a uniform palladium or palladium alloy membrane
can be formed onto the support. Generally, the OH.sup.- ion itself can
yield the desired buffering effect, so any suitable hydroxide can be used
in the subject invention as a buffering agent. For example (but not
limited thereto), the hydroxide selected from a group consisting of the
following can be employed as the buffering agent in the
palladium-containing electroplating solution of the subject invention:
sodium hydroxide, potassium hydroxide, ammonium hydroxide, and
combinations thereof. The preferred electroplating solution of the
subject invention is ammonium hydroxide.

[0028]In addition to the components described above, sulfuric acid may be
optionally added to the palladium-containing electroplating solution of
the subject invention to facilitate the dissolution of palladium sulfate.
The amount of sulfuric acid added depends on the amount of palladium
sulfate. Normally, the amount of sulfuric acid renders the concentration
of SO42- in the electroplating solution to be about 0.2 mole to
about 4 moles, and preferably, about 0.5 mole to 2 moles per liter.

[0029]The palladium-containing electroplating solution of the subject
invention can also be used to deposit a palladium alloy membrane. The
palladium-containing electroplating solution further comprises a
corresponding metal (a second metal) salt, for example, a copper salt, a
silver salt, a gold salt, a nickel salt, a platinum salt, an indium salt,
and combinations thereof. The content of the second metal salt varies
with the species of the second metal. In one embodiment of the subject
invention, the palladium-containing electroplating solution further
contains a copper salt to form a palladium-copper alloy membrane. In this
case, a copper salt such as copper sulfate or copper chloride can be
employed in an amount ranging from about 0.2 g to 100 g of copper per
liter of the electroplating solution. In the case of the addition of the
second metal, the above complexing agent, in addition to increasing the
stability of the electroplating solution, can also form a complex with a
metal that has a higher (or lower) reduction potential to decrease (or
increase) the standard reduction potential thereof. In this way, the
reduction potentials of the two metals are adjusted closer, so as to be
deposited together onto a surface of the support to form a uniform
palladium alloy membrane.

[0030]The subject invention further provides a method for providing a
palladium or palladium alloy membrane on a porous metal support,
comprising the following steps:

[0031]providing a porous metal support; and

[0032]electroplating a palladium or palladium alloy membrane on the metal
support with a palladium-containing electroplating solution, wherein said
palladium-containing electroplating solution comprises:

[0033]2 g/L to 200 g/L of palladium in a palladium salt;

[0034]10 g/L to 200 g/L of a reactive conductive salt;

[0035]10 g/L to 150 g/L of a complexing agent; and

[0036]enough buffering agent to give the electroplating solution a pH of
about 9 to 12.

[0037]In accordance with the method of the subject invention, any porous
metal supports can be used, such as (but not limited thereto) iron, an
iron alloy, copper, a copper alloy, nickel, a nickel alloy, and
combinations thereof. The iron alloy is preferred. Economically, the
porous stainless steel cataloged as the iron alloy is the electroplating
support of choice.

[0038]In the method of the subject invention, the electroplating step is
carried out under a current density ranging from about 0.01 A/dm2 to
about 1.5 A/dm2 and preferably, about 0.2 A/dm2 to about 1.0
A/dm2. The electroplating bath temperature ranges from about
40° C. to about 90° C., and preferably, about 40° C.
to about 60° C. Moreover, the metal support can be optionally
rotated during the electroplating step at a speed of not higher than 1000
rpm.

[0039]In addition to the palladium sulfate, the method of the subject
invention can also employ a palladium salt selected from a group
consisting of palladium tetrammine chloride (Pd(NH4)4Cl2),
palladium ammonium chloride (Pd(NH4)2Cl4), palladium
chloride, and combinations thereof. The content of the palladium salt in
the electroplating solution ranges from about 2 g/L to about 200 g/L (as
palladium), and preferably, about 5 g/L to about 50 g/L. The details of
the species and amount of reactive conductive palladium, complexing agent
and buffering agent can be found in the above description regarding the
palladium-containing electroplating solution of the subject invention,
and thus, are not further described herein.

[0040]In the method of the subject invention, the electroplating of the
palladium or palladium alloy membrane can be done by one electroplating
treatment using an electroplating solution with a single palladium salt
or through multiple electroplating treatments using an electroplating
solution containing two or more palladium salts. Moreover, for the
multiple electroplating treatments, the electroplating solution of each
treatment can contain the same or different palladium salts. For example,
the first electroplating treatment may be carried out with an
electroplating solution containing palladium sulfate as the palladium
salt to electroplate a thin palladium membrane on the support, followed
by a subsequent electroplating treatment with an electroplating solution
containing palladium chloride as the palladium salt to provide a
palladium membrane with the desired total thickness. In this case, due to
the relatively cheap price of palladium chloride, the preparation of the
desired plated membrane using the aforesaid two-stage electroplating
manner saves costs for electroplating of a palladium or a palladium alloy
membrane. Alternatively, the first electroplating treatment can be
carried out using an electroplating solution containing palladium
chloride as the palladium salt, followed by a subsequent electroplating
treatment using an electroplating solution containing palladium sulfate
as the palladium salt. Additionally, the second plating treatment can be
carried out with any appropriate methods, such as the electroplating
method, the electroless plating method, the vacuum sputtering method, or
the cool-rolled method.

[0041]During the electroplating process of a palladium or palladium alloy
membrane, the palladium ions accept the electrons at the cathode to
deposit onto the support as the metal Pd. Simultaneously, H2 is
generated at the cathode. Both the H2 deposits and the metal Pd on
the support cause an embrittlement susceptibility of the
palladium-containing membrane. To avoid such embrittlement susceptibility
incurred by the H2, it is possible to generate turbulence during the
palladium or palladium alloy electroplating process to mitigate or
prevent the disturbance from H2. Any appropriate means may be
employed to generate the turbulence, for example (but not limited
thereto), rotating the porous metal support as described above, and/or
producing a desired turbulence through water flow agitation, air
agitation, cathode agitation, or ultrasonic agitation. It has been found
that when a porous metal support is rotated to produce the turbulence,
the faster the support is rotated under the same current density, the
better the resulting palladium or palladium alloy membrane (that is, the
membrane exhibits a more compact lattice structure). In accordance with
the subject invention, the rotational speed of the metal support is
generally not higher than about 1000 rpm, and is preferably controlled
within a range from about 100 rpm to about 500 rpm.

[0042]In the method of the subject invention, the porous metal support can
optionally be treated with some preprocesses before the electroplating
step, such as degreasing, welding, and leveling. In particular, almost
all porous metal supports commercially available at present are stained
with greasiness thereon, which will isolate the electroplating solution
from the support and adversely affect the electroplating effect. This
isolation eventually leads to blistering, peeling or chipping of the
resulting membrane. Generally, to avoid such an adverse phenomenon, an
organic solvent such as toluene or acetone was used for cleaning the
greasiness both inside and outside the porous metal support. Subsequent
to the degreasing process, the porous metal support can also be
mechanically polished, using for example sandpaper No. 600, to remove the
work-hardening layer formed in the powder metallurgy procedure and the
oxidized layer formed in the sintering procedure involved in the
preparation of the metal support.

[0043]Moreover, the medium layer can be optionally plated onto the porous
metal support prior to electroplating the palladium or palladium alloy
membrane of the subject invention. Specifically, the medium layer can
shrink the pores of the porous metal support (i.e., filling the pores to
gradually form a smooth support surface), which is effective in providing
a compact palladium or palladium alloy membrane. Additionally, the medium
layer can improve the adhesion between the palladium or palladium alloy
membrane and the porous metal support to prevent exfoliation and thereby,
prolong the service life of the palladium or palladium alloy membrane.
For example (but not limited thereto), the medium layer can be composed
of a material selected from a group consisting of nickel, copper, silver,
gold, platinum, and combinations thereof. The preferred material for the
medium layer is nickel. Here, the medium layer can be electroplated two
or more times as desired. Meanwhile, the turbulence can be optionally
introduced into the electroplating solution during the electroplating
process to prevent the disturbance from the H2 generated therein.
The techniques of using a medium layer are described in the articles by
the following authors: Renouprez, 1 J. F. et al in Journal of Catalysis,
170, 1997, p. 181, Seung-Eun Nam in Journal of Membrane Science, 153,
1999, p. 163, Seung-Eun Nam in Journal of Membrane Science, 170, 2000, p.
91, and Journal of Membrane Science, 192, 2001, p. 177; all of which are
incorporated herein for reference.

[0044]The embodiment of the medium-layer electroplating procedure used in
the subject invention is described hereinafter with nickel used as the
medium layer. In this case, after preprocessing, the porous metal support
is placed into a plating vessel for pre-plating the nickel. Here, the
temperature of the electroplating bath ranges from about 30° C. to
about 50° C. The rotational speed of the support is about 500 rpm.
The current density ranges from about 5 A/dm2 to about 10
A/dm2, preferably from about 7 A/dm2 to about 10 A/dm2.
The electroplating duration ranges from about 3 minutes to about 6
minutes, preferably from about 4 minutes to about 5 minutes. Thereafter,
the porous metal support which has been pre-plated with nickel is washed
(e.g. by ultrasonic water rinsing), followed by a second nickel-plating
procedure in the nickel-plating vessel. In the second nickel-plating
procedure, the temperature of the electroplating bath ranges from about
30° C. to about 50° C. The rotational speed of the support
is about 500 rpm. The current density ranges from about 2 A/dm2 to
about 6 A/dm2, preferably from about 4 A/dm2 to about 6
A/dm2. The electroplating duration ranges from about 3 minutes to
about 7 minutes, preferably from about 5 minutes to about 7 minutes.
Finally, following multiple cycles of water rinsing and drying, a porous
metal support plated with a medium layer is obtained.

[0045]When the electroplating method of the subject invention is used to
provide a tube for H2 purification, the porous metal support is
optionally jointed with other metal fittings of the purification
equipment at both ends, subsequent to the degreasing process, using an
appropriate method such as argon arc welding. Then, the surface of the
porous metal support is mechanically polished as described above to
remove the work-hardening layer formed in the powder metallurgy procedure
and the oxidized layer formed in the sintering procedure during the
preparation of the metal support. The residual imprint of the previously
mentioned welding process is also removed. In this way, the porous metal
support is guaranteed to have a smooth surface to enhance the effect of
the subsequent electroplating procedure. Then, after the smooth metal
support is rinsed with water, it is ready for subsequent electroplating.

[0046]FIG. 1 shows an embodiment of the method for preparing a palladium
or palladium alloy membrane on a porous metal support in accordance with
the subject invention. As shown in FIG. 1, subsequent to the preprocesses
such as degreasing, tube welding and surface leveling, the porous metal
support is rinsed with water and optionally dried, followed by a nickel
pre-plating, a water rinsing, a nickel plating, a water rinsing, and an
optional drying step. Finally, the metal support is electroplated by
palladium, washed with water, and dried to provide a metal tube formed
from both the porous metal support and palladium membrane.

[0047]Hence, the subject invention further provides a composite with a
palladium or palladium alloy membrane, comprising:

[0051]wherein the palladium or palladium alloy membrane is substantially
free from exfoliation under the condition that the pressure at the
substrate side of the composite is up to about 3 absolute atmospheres,
preferably about 5 absolute atmospheres, and more preferably about 10
absolute atmospheres, higher than the pressure at its palladium or
palladium alloy membrane side.

[0052]In the composite of the subject invention, as described above, the
porous metal substrate can be composed of a material selected from a
group consisting of iron, an iron alloy, copper, a copper alloy, nickel,
a nickel alloy, and combinations thereof. The preferred material is an
iron alloy. Economically, the stainless steel cataloged as an iron ally
is most preferred. The medium layer interposed between the substrate and
the palladium or palladium alloy membrane can be composed of a material
selected from a group consisting of nickel, copper, silver, gold,
platinum, and combinations thereof. If the stainless steel is employed as
the substrate, nickel is preferred as the material for the medium layer.

[0053]In comparison to the prior art with the time-consuming electroless
plating method, the palladium-containing electroplating solution and the
electroplating method for preparing a palladium or palladium alloy
membrane on a porous metal support in accordance with the subject
invention can eliminate the heat treatment and reduce the preparation
time by a factor of 10. Moreover, compared to those prepared by the
electroless plating method, the palladium or palladium alloy membrane
prepared by the electroplating method of the subject invention exhibits
compact crystal grains, and when used in H2 purification components,
it is not inferior to those prepared by the electroless plating method in
terms of H2 permeability. Furthermore, the conventional palladium or
palladium alloy membranes are vulnerable to hydrogen embrittlement. In
contrast, it has been found that the palladium or palladium alloy
membrane of the subject invention is free of hydrogen embrittlement at
both low and high temperatures, and therefore has a higher applicability.
The hydrogen embrittlement of the conventional palladium or palladium
alloy membrane is related to the phase change between the palladium and
H2. The details can be found in the articles written by the
following authors: F. A. Lewis in Int. J. Hydrogen Energy, Vol. 21, No.
6, pp. 461-464, 1996, Tea-Hyun Yang et al in Electrochimica Acta., Vol.
41, No. 6, pp. 843-844, 1996, and E. Nowicka et al in Progress in Surface
Science, Vol. 48, Nos. 1-4, pp. 3-14, 1995; all of which are incorporated
herein for reference.

[0054]Exemplary embodiments are provided as follows to further illustrate
the subject invention.

EXAMPLES

Example 1

A Palladium Sulfate Electroplating Solution System

A. Preprocessing a Porous Stainless Steel Support

[0055]A porous stainless steel tube was rinsed and degreased with toluene
and acetone, and then a 15 cm long section was sliced therefrom and put
into an automatic rotational welding machine in alignment with a common
metal tube. Argon gas was injected into the tubes at a rate of 8 ml/min
to weld them together by the argon arc welding process to obtain a
support for electroplating a palladium membrane. Following the welding
process, the porous stainless steel support and its welding joint with
the common tube were mechanically polished using sandpaper No. 600 for
leveling, followed by an ultrasonic water rinsing and a subsequent drying
process in an oven at a temperature of 150° C. Then, an He stream
at 1 absolute atmosphere was injected into the support to test the gas
permeation rate out of the support. The resulting gas permeation rate was
20 L/min.

B. Electroplating of a Medium Layer

[0056]The pre-plated portion of the support had an exposed area of 50
cm2. The preprocessed support was put into a nickel pre-plating
vessel (with a radius of 120 cm and a height of 200 cm) containing 2
liters of an electroplating solution therein. The composition of the
electroplating bath and the electroplating parameters were shown in Table
1. The support was pre-plated to form a nickel coating thereon, and then
washed by an ultrasonic water rinsing process. Thereafter, the pre-plated
support was again put into a nickel-plating vessel (with a radius of 120
cm and a height of 200 cm) containing 2 liters of an electroplating
solution therein. The composition of the electroplating bath and the
electroplating parameters were shown in Table 2. Following multiple times
of water rinsing, the support was put into an oven for drying at a
temperature of 150° C., and then an He stream at 1 absolute
atmosphere was injected into the support to test the gas permeation rate
out of the support. The resulting gas permeation rate was 4 L/min.

[0057]The resulting support with a nickel medium layer was put into a
palladium electroplating vessel (with a radius of 120 cm and a height of
200 cm) containing 2 liters of an electroplating bath therein. The
composition of the electroplating bath and the electroplating parameters
were shown in Table 3. Following the electroplating process, the support
was rinsed with water many times and then dried in an oven at a
temperature of 150° C. to finally form a palladium membrane with a
compact lattice structure on the porous stainless steel support.

[0058]Steps A to C of Example 1 were repeated under the same conditions
but at a current density of 1 A/dm2 and rotational speeds of 10 rpm,
50 rpm, 100 rpm, 200 rpm, and 500 rpm. Upon the formation of the
palladium membrane, the scanning electron microscope (SEM) was used to
observe the structure of the resulting palladium membrane. FIG. 2 shows
the SEM photographs of the palladium membranes obtained at rotational
speeds of 10 rpm (A), 50 rpm (B), 100 rpm (C), 200 rpm (D) and 500 rpm
(E), respectively. It can be seen that under the same current density, a
higher rotational speed results in a more compact palladium membrane.

Example 3

A Palladium Chloride Electroplating Solution System

[0059]A palladium membrane was electroplated through steps as the same as
steps A to C of Example 1, but using the composition of the
electroplating bath and the electroplating parameters listed in Table 4.

[0060]A palladium alloy membrane was electroplated similarly through A to
C of Example 1, but using the composition of the electroplating bath and
the electroplating parameters shown in Table 5. Upon the formation of the
palladium alloy membrane, the scanning electron microscope (SEM) was used
to observe the structure of the resulting palladium alloy membrane, as
shown in FIG. 3A. Additionally, the composition of the palladium alloy
membrane was analyzed with an energy dispersive X-ray (EDX) spectrometer,
as shown in FIG. 3B.

[0061]Steps A to C of Example 1 were repeated using the composition of the
electroplating bath and conditions shown in Table 3 to electroplate a
palladium membrane on a porous metal support. The only difference is that
the electroplating lasted for 30 minutes instead. Next, the support was
taken out and rinsed with deionized water several times, and then was
electroplated with the composition of the electroplating bath and the
electroplating conditions shown in Table 4. Upon the formation of the
palladium membrane, the scanning electron microscope (SEM) was used to
observe the structure of the resulting palladium membrane, as shown in
FIG. 4.

Example 6

Test on He Permeability

[0062]At room temperature, the porous metal support tube with a palladium
membrane (referred to as the "membrane tube" hereinafter) obtained from
Example 1 was filled with He at 4 absolute atmospheres, and put into a
water bath to observe the compactness of the membrane tube. It was found
that the He could not penetrate through to the outside of the membrane
tube. This meant that the membrane tube could withstand a 4-absolute
internal pressure of He.

Example 7

Test on Ar Permeability

[0063]An apparatus shown in FIG. 5 was utilized in this example. The
membrane tube (2) obtained from Example 1 was placed into a shell and
tube reactor (3). At room temperature, Ar was introduced into the reactor
(3) via a gas inlet (1). The outside outlet (5) of the membrane tube was
opened so the reactor could be filled with Ar. Then, the outside outlet
(5) was closed to build up a backpressure inside the reactor. When the
backpressure reached 10 absolute atmospheres, an observation was made in
the inside outlet (4) of the membrane tube to check if any Ar had
permeated through the pores of the membrane tube into the interior
thereof. The test results showed that no Ar from the reactor (3) had
permeated through the membrane tube (2) into the interior thereof. This
meant that the membrane tube (2) could withstand an external Ar pressure
of 10 absolute atmospheres safely.

Example 8

Test on H, Permeability

[0064]Similarly, the apparatus shown in FIG. 5 and the membrane tube (2)
obtained from Example 1 were used in this example. At room temperature,
Ar was introduced into the reactor (3) via the gas inlet (1). The outside
outlet (5) of the membrane tube was opened so that the reactor (3) could
be filled with Ar. Then, the temperature of the reactor (3) was increased
from room temperature to 380° C. at a rate of 2.5° C./min,
while the inlet gas was replaced with H2 of industrial level. When
the residual Ar was purged completely from the reactor (3) by H2, a
regulating valve (7) on the outside outlet (5) was adjusted to maintain a
pressure of 5 absolute atmospheres inside the reactor. Under such a
pressure difference, H2 was driven to permeate through the
membrane-plated tube (2) to the inside outlet (4) of the membrane tube,
where a permeation ratio of H2 was measured to be 727 ml/min. Then,
the regulator valve (7) on the outside outlet (5) was adjusted to
maintain a pressure of 10 absolute atmospheres inside the reactor (3), in
which case the permeation ratio of H2 was measured to be 1481
ml/min. These results demonstrated that the palladium membrane prepared
by the subject invention exhibited an excellent H2 permeability.

Example 9

Hydrogen Embrittlement Test

[0065]A welded porous stainless steel tube was mechanically polished using
sandpaper No. 600, and then was dipped into 10 moles of hydrochloric acid
for 3 to 5 minutes and rinsed with deionized water. Subsequently, the
resulting tube was immersed into a tin chloride sensitizing solution for
5 minutes, and then was immersed into deionized water for 2 minutes. The
tube was then immersed into a palladium chloride activator for 5 minutes
and again into deionized water for another 2 minutes. Such a cycle was
repeated ten times, after which the activated tube was immersed into an
electroless plating solution (comprising palladium ammonium chloride and
a reducer hydrazine) to obtain a porous stainless steel tube with a
palladium membrane prepared by the electroless plating method (referred
to as the "electroless-plated palladium membrane tube" hereinafter).

[0066]Then, the electroless-plated palladium membrane tube and the tube
obtained from Example 1 were subjected to the hydrogen embrittlement test
at room temperature. At first, H2 was injected into the
electroless-plated palladium membrane tube to attain a pressure of 3
absolute atmospheres. As shown in FIG. 6, the hydrogen embrittlement and
chipping phenomena occurred in the electroless plated palladium membrane.
Then, H2 was injected to the electroplated palladium membrane tube
of the subject invention to attain a pressure of 3, 5, and 10 absolute
atmospheres, respectively. No hydrogen embrittlement was found.

[0067]Next, the operation temperature was increased until the phase change
temperature of palladium, i.e., about 250° C. to 300° C.,
was reached. The electroplated palladium membrane tube of the subject
invention was tested again by introducing H2 to attain a pressure of
3, 5, and 10 absolute atmospheres, respectively. After six hours under
the phase change temperature, slow gas leakages were observed in the
electroplated palladium membrane tube, but still no chipping occurred, as
shown in FIG. 7.

Example 10

H2 Purifying Test

[0068]Similarly, the apparatus shown in FIG. 5 and the membrane tube (2)
obtained from Example 1 were used in this example. At first, the
regulating valve (7) on the outside outlet (5) of the membrane tube was
adjusted to maintain a normal pressure inside the reactor (3), and a gas
mixture comprising 75% of H2 and 25% of CO2 was used as the
feeding gas to test the H2 purity which could be obtained by the
membrane tube (2). With a continuous injection of the gas mixture into
the reactor (3), the regulating valve (7) on the outside outlet (5) was
further adjusted to maintain a pressure of 5 absolute atmospheres inside
the reactor (3). Under the resulting pressure differential, H2 in
the reactor (3) permeated through the membrane tube (2) to the inside
outlet (4) of the membrane tube. In this case, an H2 flow rate of
326 ml/min was measured on the outside outlet of the membrane tube (2)
with a purity higher than 99.997% (CO, CO2 and CH4 in
concentrations of lower than 10 ppm). The H2 flow rate measured on
the outside outlet (5) of the membrane tube was 465 ml/min and the
content of H2 was decreased from 57.5% to 75%, which represented a
recovery rate of 55%.

[0069]Subsequently, the regulating valve (7) on the outside outlet (5) was
further adjusted to maintain a pressure of 10 absolute atmospheres inside
the reactor (3). Under the resulting pressure differential, the H2
in the reactor (3) permeated through the membrane tube (2) to the inside
outlet (4) of the membrane tube. In this case, the H2 flow rate of
649 ml/min was measured from the outlet of the membrane tube with a
purity higher than 99.997% (all CO, CO2 and CH4 in a
concentration of lower than 10 ppm). The H2 flow rate measured on
the outside outlet (5) of the plated tube was 794 m/min and the content
of H2 was reduced from 54.6% to 75%, which represented a recovery
rate of 60%.

Example 11

[0070]As shown in FIG. 8, a steam reforming reactor (8) and a palladium
membrane tube reactor (9) (including the membrane tube obtained from
Example 1) were connected in series. At a rate of 2.5° C./min, the
temperature of the steam reforming reactor (8) was increased from room
temperature to 280° C., while that of the palladium membrane tube
reactor (9) was increased from room temperature to 350° C. When
the temperature rose, Ar was injected into the reactor as a protective
gas. Once the temperature settings were reached, a liquid mixture of
methanol and water was supplied by a pump (10) so that the methanol
reacted with water in the steam reforming reactor (8) to produce H2
and CO2. Then, the resulting gas mixture was subjected to an H2
purifying process for separation by passing through the palladium
membrane tube reactor (9). A regulating valve (12) was adjusted to
maintain a pressure of 10 absolute atmospheres inside the palladium
membrane tube reactor (9). As a result, an H2 permeation rate of 30
liters per hour was measured with an H2 purity of 99.95%.

[0071]The above examples are intended to exemplify the embodiments of the
subject invention and illustrate the technical features thereof, but not
to limit the scope of protection of the subject invention. Any
modifications or equivalent replacements that can be easily accomplished
by persons skilled in the art are within the scope of the subject
invention. The scope of the protection of the subject invention is based
on the following claims as appended.